Oxygen assisted ohmic contact formation to N-type gallium arsenide

ABSTRACT

This invention describes a low resistance contact structure to n-type GaAs and a method for making such a contact structure. The contact structure is formed by depositing successive layers of Ni, Au, Ge, and Ni. A fifth layer is then deposited on the first four layers. The fifth layer is a metallic tungsten oxide. The metallic tungsten oxide is formed by sputtering tungsten onto the 4 layer stack in a low pressure argon plus oxygen atmosphere. The resulting 5 layer stack is then annealed in a rapid thermal anneal (RTA) process. The RTA process heats the stack for 5 seconds at 600 degrees. The resulting structure consists of an intermetallic NiGe compound having a small amount of a AuGa compound dispersed within it and being covered by a metallic tungsten oxide film. The oxygen from the metallic tungsten oxide film acts as a gettering mechanism to create gallium vacancies in the GaAs lattice structure during the RTA process. The oxygen forms a compound with gallium which is sandwiched between the metallic tungsten layer and the NiGe metallurgy. The sheet resistance of the contact metallurgy is low because the metallic tungsten oxide film is substantially thicker than that required to provide oxygen for the gettering process. The contact resistance to the n-type GaAs is low because the oxygen acts in a similar fashion to gold and creates more gallium vacancies in the GaAs. These vacancies are filled with an n-type dopant (Ge), supplied by the contact metallurgy, to create a better ohmic contact. The contact structure is reliable because there is a low gold content in the contact and because the nickel stabilizes the germanium which is not used for filling the gallium vacancies in the GaAs lattice.

FIELD OF THE INVENTION

This invention relates to metal/semiconductor contact structures and tomethods for manufacturing such structures. More particularly, theinvention relates to metal/semiconductor contact structures to n-typegallium arsenide. Specifically, the invention discloses the use ofmetallic tungsten oxide in conjunction with a low gold content nickelgermanium metallurgy, to obtain a low resistance thermally stable ohmiccontact to n-type gallium arsenide.

BACKGROUND OF THE INVENTION

Electrical devices built from gallium arsenic (GaAs) offer manyadvantages over conventionally built silicon semiconductor devices. Inparticular, gallium arsenide devices operate at higher speeds thansilicon devices. This is due to the fact that electrons have a highermobility in gallium arsenide than in silicon. However, the speedadvantage inherent in gallium arsenide cannot be realized unless theelectrical signals produced by the individual devices can be transmittedto other devices. Gallium arsenide devices, like silicon devices, mustbe connected to other devices without impairing the signal transmissionbetween devices. Therefore, transmission of signals to and from thedevices requires high quality contacts. This is a particular problem forgallium arsenide because contacts are not as easily made to galliumarsenide devices as they are to silicon devices. Therefore, unless highquality contacts can be made to gallium arsenide, the speed advantage ofgallium arsenide over silicon will not be realized.

The electrical quality of the contact to the gallium arsenide device ismeasured by the resistance (called contact resistance) between thegallium arsenide semiconductor and the contact metallization. If thecurrent flow through the contact is linear with the applied voltageacross the contact and the resistance (V=IR) is low, the contact is saidto be a good ohmic contact. Typically, the various gallium arsenidedevices are interconnected during circuit fabrication by aluminum alloywiring, although other types of metals can be used. However, merelydepositing the aluminum (or most other metals) directly onto the galliumarsenide creates a Schottky diode rather than an ohmic contact. Thisdiode can be made into an ohmic contact to n-type GaAs by doping theGaAs with an n-type dopant. Specifically, the dopant concentration inthe GaAs must be in excess of approximately 1E19 per cubic centimeter tocreate an ohmic contact. This level of doping creates enough tunnellingcurrent through the diode potential barrier that the diode behaveselectrically like an ohmic contact. The problem with this solution toGaAs n-type contacts is that conventional doping and annealingtechniques of GaAs are not sufficient to dope GaAs to such high levels.Therefore, in order to create an ohmic contact to GaAs, the metallurgyitself must supply the necessary dopants.

Prior art attempts to solve the problem of GaAs contact resistance havegenerally included a germanium-gold (Ge-Au) alloy layer interposedbetween the n-type gallium arsenide and the wiring metallurgy. Thismulti-element alloy is employed to facilitate the incorporation of adopant into the GaAs lattice. A commonly used ohmic contact to GaAs isbased on the eutectic germanium-gold (Ge-Au) alloy (88 weight % Au), inconjunction with nickel (Ni). The compound is typically annealed abovethe eutectic temperature of approximately 360 degrees Celsius whichresults in the alloy being melted. The gold is highly reactive with Gafrom the GaAs lattice and forms Au-Ga compounds during the annealingprocess. This leaves Ga vacancies in the GaAs lattice which are occupiedby Ge, an n-type dopant. This n-type dopant contributes to thetunnelling current and helps form an ohmic contact even though thedoping concentration without the contact metallurgy is less than1E19/cubic centimeter. The problem with this solution to the contactproblem is that, in addition to having low resistance, contacts must bethermally stable. Device processing after the contact formation includesmultilevel interconnect processing and packaging. These steps can exposethe GaAs device to temperatures of approximately 400 degrees Celcius forperiods of a few minutes to several hours. Heating the eutectic basedGe-Au-Ni contacts to 400 degrees after contact formation results inlocal melting of the contact which raises the contact resistance. Inaddition to the contact resistance being raised generally, the variancein contact resistance between devices on a chip is also raised. This isbecause the melting of the contact metallurgy does not occur uniformlyacross the chip. Therefore, the eutectic based Ge-Au-Ni contact is not asolution to the GaAs contact resistance problem because the processesafter the contact formation degrade the Ge-Au-Ni compound contactresistance. Another prior art attempt to produce a reliable contactstructure to gallium arsenide comprises a plurality of layers ofdifferent metals. Specifically, the contact structure includes a firstlayer of nickel covered by a thin layer of gold followed by a Ge-Nilayer and a tungsten (W) layer. The gold content in the annealedNiGe(Au)W compound is low compared to the eutectic Ge-Au-Ni compound.The NiGe(Au)W compound is more thermally stable than the eutecticGe-Au-Ni compound and it still retains enhanced contact resistance. Thetungsten is added in order to reduce the sheet resistance of the low Aucontent alloy. The resulting contact structure is characterized by a lowcontact resistance which has high thermal stability, smooth morphologyand a uniform metal to gallium arsenide interface. These characteristicsare essential for providing the improved reliability of the metal togallium arsenide contact structure. The problem with this particularcontact structure, however, is that tungsten has a very high meltingpoint. Therefore, evaporating tungsten onto gallium arsenide isincompatible with VLSI processing of high performance gallium arsenidedevices. Furthermore, conventional sputtering of the tungsten onto theidentical nickel/germanium/gold layered structure results in contactswith poor contact resistance. Sputtering the tungsten, rather thanevaporating it, defeats the purpose of the contact structure.

OBJECTS OF THE INVENTION

It is an object of the present invention to manufacture a contact togallium arsenide.

It is a further object of the present invention to manufacture a contactto gallium arsenide having a low contact resistance.

It is still another object of the present invention to manufacture acontact to gallium arsenide having low contact resistance variance.

It is still a further object of the present invention to manufacture acontact to gallium arsenide having high reliability.

It is still another object of the present invention to manufacture acontact to gallium arsenide having thermal stability.

It is still a further object of the present invention to manufacture acontact to gallium arsenide which is compatible with VLSI processing ofhigh speed devices.

SUMMARY OF THE INVENTION

This invention describes a low resistance contact structure to n-typeGaAs and a method for making such a contact structure. The contactstructure is formed by depositing successive layers of Ni, Au, Ge, andNi. A fifth layer is then deposited on the first four layers. The fifthlayer is a metallic tungsten oxide. The metallic tungsten oxide isformed by sputtering tungsten onto the 4 layer stack in a low pressureargon plus oxygen atmosphere. The resulting 5 layer stack is thenannealed in a rapid thermal anneal (RTA) process. The RTA process heatsthe stack for 5 seconds at 600 degrees. The resulting structure consistsof an intermetallic NiGe compound having a small amount of a AuGacompound dispersed within it and being covered by a metallic tungstenoxide film. The oxygen from the metallic tungsten oxide film acts as agettering mechanism to create gallium vacancies in the GaAs latticestructure during the RTA process. The oxygen forms a compound withgallium which is sandwiched between the metallic tungsten layer and theNiGe metallurgy. The sheet resistance of the contact metallurgy is lowbecause the metallic tungsten oxide film is substantially thicker thanthat required to provide oxygen for the gettering process. The contactresistance to the n-type GaAs is low because the oxygen acts in asimilar fashion to gold and creates more gallium vacancies in the GaAs.These vacancies are filled with an n-type dopant (Ge), supplied by thecontact metallurgy, to create a better ohmic contact. The contactstructure is reliable because there is a low gold content in the contactand because the nickel stabilizes the germanium which is not used forfilling the gallium vacancies in the GaAs lattice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the contact structure of the present invention beforethe RTA process step.

FIG. 2 illustrates the energy band relationship of the presentinvention.

FIG. 3 illustrates the contact resistance of the present invention as afunction of anneal temperature.

FIG. 4 illustrates the sheet resistance of sputtered tungsten oxide as afunction of oxygen partial pressure.

FIG. 5 illustrates the structure of the present invention after the RTAprocess step.

FIG. 6 illustrates the gallium and oxygen content of the presentinvention as a function of depth into the contact stucture.

FIG. 7 illustrates the gallium and oxygen content of a prior art contactstructure as a function of depth into the contact structure.

FIG. 8 illustrates the contact resistance of the present invention as afunction of anneal temperature in comparison to a non-gettering tungstenalloy.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates the contact structure of the present invention. Thegallium arsenide substrate 10 has a donor doped impurity region 20. Thecontact to the donor doped impurity region is deposited as a five layerstructure. The first layer 32 is a nickel layer. The second layer 34 isa gold layer. The third layer 36 is a germanium layer. The fourth layer38 is a nickel layer. The fifth layer 40 is crucial to the contactresistance between the gallium arsenide and the contact metallization.The fifth layer 40 is a metallic tungsten oxide rather than a puretungsten film. Once the five layers have been individually deposited,the contact structure is thermally annealed. The anneal forms a metalalloy 25 (FIG. 5) covered by the metallic tungsten oxide film 40.Aluminum wiring 42 is then deposited on the contact structure tocomplete the interconnection of the GaAs device to other devices as islater explained with respect to FIG. 5.

FIG. 2 illustrates a theoretical energy band structure for the presentinvention with respect to the depth (X) of the contact structure. TheFermi level (Ef) in the GaAs lattice is equal in energy (electron volts)to the Fermi level of the metal alloy contacting the GaAs at theinterface between the metal alloy and the GaAs at thermal equilibrium.Phi is the barrier height at the metal alloy/GaAs interface. Theconduction band energy Ec equals the Fermi level plus the barrierheight. The conduction band energy in the GaAs away from the metal/GaAsinterface is equal to that of the bulk conduction band energy in GaAs.Also, heavy doping in the N++ region makes the GaAs degenerate so thatthe Fermi level is above the conduction band in the N++ region. Theconduction band energy Ec minus the valance band energy Ev in GaAs mustalways be constant. As a result, the valance band energy follows theconduction band energy with respect to depth in the GaAs. FIG. 2illustrates how the conduction and valance band energy levels bendupward in energy near the metal/GaAs interface due to the aboveconstraints. This bending of the energy bands creates an energy wall 50which is thin enough in the depth direction (X) that electrons cantunnel through it as shown at 52, as opposed to gaining enough energy togo over the energy wall as shown at 54. The theoretical current densityJt which results from electrons tunnelling through the energy bands isshown in FIG. 2. This current density from the tunnelling electronsincreases exponentially with the square root of the donor dopingconcentration Nd in the GaAs.

In order to create the tunnelling of electrons between the contactmetallurgy and the n-type GaAs, gallium vacancies must be created in theGaAs lattice structure and filled with an n-type donor element. Thevacancies in the GaAs lattice structure are created by annealing goldover the GaAs semiconductor. The vacancies are filled by annealing goldand GaAs in the presence of an n-type dopant element (with respect tothe III-V GaAs lattice structure) such as germanium. Further, thegold/germanium/GaAs reaction is better controlled by using a first layerof nickel, which has a thickness of approximately 50 Angstroms. Thefirst nickel layer facilitates the Au-Ga reaction by penetrating througha native oxide which may be present on the GaAs surface. Thispenetration improves the lateral uniformity of the AuGa reaction. Thegold layer thickness is approximately 60 Angstroms. A germanium layer ofapproximately 275 Angstroms covers the gold layer, and is in turn,covered by a second nickel layer of approximately 125 Angstroms. Thesecond nickel layer acts to provide thermal stability to the resultingcontact by reacting with the Ge atoms which do not diffuse into theGaAs. The melting point of this intermetallic NiGe compound isapproximately 750 degrees Celsius which is in excess of any subsequentprocessing temperatures. Finally, the thin gold layer is placed belowthe germanium layer, as opposed to over the germanium layer, to improvethe gettering of the gallium from the GaAs lattice.

The thickness of the first nickel layer in the contact structure mayrange from approximately 0-50 Angstroms. The thickness of the gold layermay range from approximately 40-60 Angstoms. The thickness of thegermanium layer in combination with the second nickel layer is adjustedso as to form the NiGe compound. As deposited, the combined separatelayers have a thickness of 400 Angstroms with 275 Angstroms being Ge and125 Angstroms being Ni. The total NiGe layer may be much thicker orthinner as long as the ratio of the Ge to Ni thickness remainsapproximately 2.2 to 1. After the deposition of the Ni/Au/Ge/Ni layers,they are subsequently covered with a fifth layer and then annealed. Theannealing step is carried out by a rapid thermal anneal process (RTA).The RTA process ramps up the temperature of the anneal at approximately100 degrees per second. The heating is done by a quartz lamp bank in anargon atmosphere. The equipment for the process is well known andcommercially available. The anneal is done at 600 degrees (Celsius) forapproximately 5 to 30 seconds. The proper anneal can be accomplished atdifferent temperatures for different times. In particular, a differentNiGe layer thickness from the preferred 275 Angstroms Ge plus 125Angstroms Ni will require an adjustment in the anneal conditions withthicker films requiring longer anneal times.

The fifth layer of material which covers the Ni/Au/Ge/Ni layers isparticularly critical to this invention. This is because the fifth layeralso helps to getter gallium from the GaAs lattice as a complement tothe gold which is already in the metal alloy. While gold is useful as agettering agent, a large amount of gold (such as in the eutectic basedcontacts) in the contact degrades the reliability of the contact.Further, adding gold to the present contact metallurgy beyond a minimumamount does not substantially increase the gettering of the gallium fromthe GaAs lattice. The fifth layer is important because it adds oxygen tothe contact metallurgy without creating a detrimental oxide. Addingoxygen to the metallurgy is important because oxygen acts like gold inthat it getters gallium from the GaAs lattice when the contact structureis annealed, but unlike gold it does not degrade the contact underthermal stress. The oxygen getters gallium which is in addition to thatgettered by the gold. Therefore, more vacancies in the GaAs latticestructure are created which leads to more doping of the GaAs latticewhich, in turn, lowers the contact resistance to the GaAs.

The fifth layer is a reactively sputtered metallic tungsten oxide film.The fifth layer is W_(x) O, where x is between approximately 2.5 and 9,preferably 4. The tungsten is a mixture of beta tungsten (A-15structure) and an amorphous phase of tungsten which are both stabilizedby oxygen. The tungsten film is reactively sputtered onto theNi/Au/Ge/Ni layers in an atmosphere having a mixture of an inert gas andoxygen. The inert gas is typically argon but may also be Xenon, Krypton,or Neon. The oxygen is mixed with the Argon in a controlled fashion. Theoxygen reacts with the sputtered tungsten to form a metallic tungstenoxide (W_(x) O) film. Sputtering pure tungsten is a well known processand is carried out at room temperature. As a result, the sputteringtechnique is well suited for high performance GaAs device fabrication.The added oxygen content in the sputtered tungsten is low enough so thatthe sheet resistance of the film is low, yet high enough to lower thecontact resistance. FIG. 3 illustrates the effect (on contactresistance) of introducing oxygen into the tungsten fifth layer of theGaAs contact structure.

FIG. 3 plots the average contact resistance to n-type GaAs regions as afunction of anneal temperatures for different contact structures. Plot Arepresents the contact resistance for sputtered tungsten in an argononly atmosphere. Plot B represents the same parameter for sputteredtungsten in an argon plus oxygen atmosphere. A comparison between thetwo plots shows not only that the average minimum contact resistance hasdecreased by a factor of 3 (from 0.45 to 0.15 ohm-mm), but that the dataspread in contact resistance has also substantially decreased. Both ofthese improvements are important for VLSI GaAs applications. This isbecause decreasing the contact resistance increases the speed of deviceand associated circuitry. Additionally, many circuits depend on closedevice parameter tracking in order to generate the correct signals.Decreasing the contact resistance spread improves the device parametertracking, and therefore, the ability to generate the proper electricalsignals.

The metallic tungsten oxide film as deposited is approximately 500 to1000 Angstroms thick. The thickness of the film controls the sheetresistance of the contact. FIG. 4 illustrates the sheet resistance ofthe W_(x) O as a function of the oxygen (O₂) partial pressure in thesputtering process for a 1000 Angstroms thick film on quartz substrates.The partial pressure of oxygen used in curve B of FIG. 3 was 1.5 E-4 in1E-3 Torr of argon. The ratio between oxygen and argon gas pressureswill vary when different sputtering techniques or deposition geometriesare employed. However, adjusting the oxygen partial pressure withrespect to the argon pressure to produce W_(x) O films with a sheetresistance of between approximately 50 to 175 ohms per square,preferably 135 ohms per square for a 1,000 Angstrom thick film, yieldsthe proper oxygen level in the tungsten layer. Controlling the sheetresistance of the metallic oxide film is an easier and more accuratemethod of forming the W_(x) O film than determining the correct oxygenflow from physical analysis of individual deposition systems. Once themetallic tungsten oxide layer has been deposited on the Ni/Au/Ge/Nilayers, the five layer structure is subjected to the RTA process.

FIG. 5 illustrates the contact structure after it has been annealed. Thefirst annealed layer 25 is composed of NiGe having a small amount ofAuGa within it. The amount of AuGa in the NiGe layer is less than 1% ofthe AuGa found in annealed eutectic-based Au-Ge-Ni contacts. The secondlayer is the metallic tungsten oxide layer 40 which is depleted ofoxygen in its lower portion 27 (adjacent the first layer 25). The lowerportion of the metallic tungsten oxide layer 27 which is depleted ofoxygen has a thickness of approximately 50-100 Angstroms. The oxygenwhich has outdiffused from layer 40 has reacted with Ga to create a thindiscontinuous Ga₂ O₃ layer 26 within the first annealed layer 25. TheGa₂ O₃ compound may be found as close as 300 Angstroms from the GaAssurface. Additionally, the first annealed layer 25 has penetrated intothe GaAs substrate. The amount of penetration 28 from the surface of theGaAs is less than 500 Angstroms. The aluminum alloy wiring metallurgy 42is then deposited onto the annealed contact structure in a conventionalmanner.

The process of creating a metallic tungsten oxide film can beaccomplished in several different ways. For example, it can be performedin a plasma enhanced chemical vapor deposition (PECVD) system with a13.56 MHz plasma generated in a mixture of WF₆, O₂, and H₂.Additionally, the metallic tungsten oxide film need not have a uniformcomposition throughout. The important limitation is that there be enoughoxygen in the lower portion 27 of the W_(x) O film so that the contactresistance is reduced through the formation of a gallium oxide compound.The composition of the metallic tungsten oxide film above the lowerportion 27 of the film can have much less or no oxygen in it. That is,pure tungsten can be the material above the lower portion of the W_(x) Ofilm and the contact resistance will still be effectively reduced.

The fact that introducing oxygen into the metallurgy promotes getteringof gallium is illustrated in FIGS. 6-8. FIG. 6 is an Auger analysis,after annealing, of the contact structure having a metallic tungstenoxide fifth layer. FIG. 7 is the same analysis of a contact structurehaving only sputtered tungsten as a fifth layer. Finally, FIG. 8illustrates the effect of substituting other elements for oxygen in thefifth layer. FIG. 6 illustrates that the oxygen content for the W_(x) Ometal alloy sputtered in the argon plus oxygen atmosphere isapproximately 25%, whereas FIG. 7 shows that the oxygen content for theW metal alloy sputtered in argon is less than 1%. The increased oxygencontent in the W_(x) O layer is expected. However, FIG. 6 also showsthat, near the metal alloy/semiconductor interface, there is a galliumpeak which coincides with an oxygen peak. This shows that agallium-oxygen compound is forming near the metal alloy/semiconductorinterface. That is, the gallium is being gettered out of the GaAslattice structure by oxygen diffusing out from the W_(x) O film andforming a Gu₂ O₃ compound. The sheet resistance of the W_(x) O film isnot affected by the outdiffusion of oxygen because only a smallthickness, approximately 50-100 Angstroms, is depleted of oxygen duringthe anneal. FIG. 7 illustrates that when there is very little oxygen inthe structure, the gallium does not substantially outdiffuse. Therefore,oxygen is the element which promotes the increased outdiffusion ofgallium. The outdiffusion of gallium as a result of the introduction ofoxygen and its effect on contact resistance is not an expected effect.Further, FIG. 8 illustrates the contact resistance of separate contactstructures as a function of anneal temperature for a W_(x) O fifth layercompared to a tungsten nitride (W_(x) N) compound used a fifth layer.Again, the W_(x) O layer has a low contact resistance when compared withthe alloy using tungsten nitride as a fifth layer. Nitrogen is not asreactive with gallium as is oxygen regardless of the anneal temperatureand the contact resistance of the tungsten nitride structure is notreduced to the level of the metallic tungsten oxide structure.Therefore, nitrogen does not getter the Ga from the GaAs lattice and isnot a replacement for oxygen in the contact structure.

Although the oxygen cannot be replaced by nitrogen in the contactstructure to GaAs, substitutions for other elements in the contactstructure can be made. Specifically, silicon (Si) could be substitutedfor germanium. This would provide for a Ni-Si-Au-Ni metal stack on whichthe W_(x) O is deposited. When five layer stack is annealed, the siliconwould occupy the lattice sites left empty by the gallium outdiffusiondue to the oxygen. Also, palladium (Pd) could be substituted for thenickel layers. Palladium penetrates through any native oxide on the GaAssurface and is thermally stable in a compound with germanium or silicon.Finally, nickel oxide could replace metallic tungsten oxide as thesource of the oxygen in the metallurgy system. Nickel has a lowresistance and forms a suitable oxide compound for sputtering on thefour layer stack. When nickel oxide is the source for the oxygen in thecontact structure, it may be possible to reduce the thickness of thenickel layer separating the germanium (or silicon as the case may be)from the nickel oxide layer. When the types of layers are modified asdescribed above, the thicknesses of those layers may also have to bemodified as well as the anneal temperatures. However, thesemodifications will be well understood by those skilled in the art.

While this invention has been particularly described and illustratedwith references to particular embodiments thereof, it will be understoodby those skilled in the art that changes in the above description orillustrations may be made with respect to former or detail withoutdeparting from the spirit or scope of the invention.

Having thus described our invention what we claim as new and desire tosecure as Letters Patent, is:
 1. A contact to gallium arsenidecomprising:a first metal alloy layer having a gallium compound andincluding gold, a thermally stable element, and a dopant element, saidfirst metal alloy layer contacting said gallium arsenide; a second layerconsisting essentially of a metal element chosen from a group consistingof tungsten or nickel and oxygen; said first metal alloy layerinterposed between said gallium arsenide and said second layer; and athird layer interposed between said first and second layers, said thirdlayer consisting essentially of a gallium oxide compound.
 2. A contactto gallium arsenide, as in claim 1 wherein:said second layer contains 10to 30 atomic percent oxygen.
 3. A contact to gallium arsenide, as inclaim 1 wherein:said thermally stable element is chosen from the groupconsisting of palladium or nickel, and said dopant element is chosenfrom the group consisting of germanium or silicon.
 4. A contact togallium arsenide, as in claim 3 wherein:said first layer comprisesnickel, germanium, and gold.
 5. A contact to gallium arsenide, as inclaim 4 wherein:said second layer contains 10 to 30 atomic percentoxygen.
 6. A contact to gallium arsenide comprising:a first layerconsisting essentially of a gallium oxide compound; a second layerconsisting essentially of a metal element and oxygen; a third layercomprising nickel, germanium, and gold; said first layer interposedbetween said second and third layers; and said third layer contactingsaid gallium arsenide.
 7. A contact to gallium arsenide, as in claim 6wherein:said metal element is chosen from the group consisting of nickeland tungsten.
 8. A contact to gallium arsenide, as in claim 7wherein:said third layer comprises 50 atomic percent nickel, 40 atomicpercent germanium, and 10 atomic percent gold.
 9. A contact to galliumarsenide, as in claim 8 wherein:said second layer contains 10 to 30atomic percent oxygen.
 10. A contact to gallium arsenide, as in claim 6wherein:said second layer contains 10 to 30 atomic percent oxygen.